1. Field of the Invention
This invention relates to systems that scan images, more particularly to such systems that use spatial light modulators.
2. Background of the Invention
Systems that scan images can be found in many places. Supermarkets use them to scan bar codes on product labels. These are normally mounted underneath the counter top, and the system scans the bar code from below. Department stores typically have a hand held unit that allows the clerk to scan the tags of products being purchased. When inventory records are due, the clerk can walk out onto the floor and record items hanging on the racks. Scanning is also used in many other applications such as edge scanners for automated material movement.
Conventional scanners normally utilize a mirror that is used in conjunction with a light source. In supermarket scanners, for example, the light source is some type of laser. The mirror is a rotating polygonal solid which scans a dot of light along a linear axis. There is a detector, normally mounted near the scanning optical system that records the light bouncing back into the unit. When the bar code has a white area, there is a low level of light returned, since white paper is a light diffuser. But when the light dot hits a black bar, the light level drops. This drop off signals the unit to do some simple processing. It counts the increment of time for which there is no return light. A thick black bar may be six or seven increments, a thin one only one or two. After the bar code is scanned in this manner, the cash register then compares the read bar code with its database of products, selects the product and its price.
Occasionally, this systems take quite of bit of time to read a bar code. The system is still scanning at the same rate, but because of the motion of the mirror, occasionally there is a mis-read. The cash register cannot find the related product, and the code must be read again. More importantly, mechanical drivers such as the one to spin the mirror are expensive and consume a relatively large amount of power. Additionally, the units must be somewhat bulky to allow for the mirror to have enough room to spin, and the coil or motor that give the impetus for the mirror to move. Finally, these mechanical parts wear out and start giving consistently bad readings as the mirror does not spin correctly, or they just stop functioning completely.
It would be desirable to eliminate the moving mechanical parts, to increase each units lifetime. This would also serve to make the units lighter and more accurate, eliminating the need for re-scans.